The present invention relates to a photoacoustic signal detecting method and apparatus for detecting information relative to the surface and inside of a sample using photoacoustic effect, and a method for detecting internal defects of a semiconductor device.
The photoacoustic effect, which was discovered by Tyndall, Bell, Rontgen, et al. in 1881, represents the following phenomenon. When, as shown in FIG. 23, intensity-modulated light (intermittent light) 19 is irradiated to a sample 7, through a lens 5, heat is generated in a light absorption region Vop 21 and periodically diffused through a heat diffusion region Vth 23 defined by thermal diffusion length .mu.s so that the thermal distortion wave thus generated provides a surface acoustic wave (ultrasonic wave). By detecting this ultrasonic wave i.e. a photoacoustic wave by a microphone (acoustic-electric converter) or a piezo-electric element to obtain the component in synchronism with the incident light, information relative to the surface and inside of the sample can be obtained. A technique for detecting the above photoacoustic signal is disclosed in "HIHAKAI KENSA", Vol. 36, No. 10, pp. 730-736, October 1987 (Showa 62) or IEEE; 1986 ULTRASONIC SYMPOSIUM--pp. 515-526 (1986). Now referring to FIG. 22, this one example of technique will be explained in the case where a laser is used as a light source.
A parallel light emitted from a laser 1 is intensity-modulated by an acoustic-optical modulator element (AO converter) 2. The thus obtained intermittent light is expanded to a beam of a desired diameter by a beam expander 3, which is reflected by a beam splitter or half mirror 4 and thereafter focused on the surface of a sample 7 placed on an XY stage 6 by a lens 5. Then, the heat distortion wave created at a focusing position 21 generates an ultrasonic wave and also provides a minute displacement in the sample surface. This minute displacement will be detected by a Michelson interferometer explained below. A parallel light emitted from a laser 8 is expanded to a beam of a desired diameter by a beam expander 9. This beam is separated into two optical paths by a beam splitter or half mirror 10. The one is focused on the focusing position 21 on the sample 7 by a lens 5 whereas the other is irradiated to a reference mirror 11. Then, the light reflected from the sample 7 and the light reflected from the reference mirror 11 interfere with each other on the beam splitter 10. The interference pattern thus formed is focused on a photoelectric converting element 13 (e.g. photodiode) through a lens 12 to provide a photoelectric-converted interference intensity signal. This interference intensity signal is amplified by a preamplifier 14 and thereafter sent to a lock-in amplifier 16. The lock-in amplifier 16, using as a reference signal a modulated frequency signal from an oscillator 15 used for driving the acoustic-optical modulation element 2, extracts only the modulated frequency component contained in the interference intensity signal. This frequency component has information relative to the surface or inside of the sample according to the frequency. By varying the modulated frequency, the thermal diffusion length .mu.s 21, the information in a direction of the depth of the sample can be obtained (FIG. 23).
The inventors of the present invention, by improving the photoacoustic signal detecting apparatus as described above, disclosed the device with enhanced precision of detecting the information relative to the surface and inside of a sample in USSN 384541 filed July 24, 1989 entitled "Photoacoustic Signal Detecting Device" and assigned to the same assignee as the present application. In accordance with one aspect of the invention of this prior application, both focusing means for intensity-modulating the light emitted from a first laser light source to be focused on a sample, and interferometry detection means for detecting photoacoustic effect through interferometry and constructed in a confocal optical system so that the transverse resolution and detecting sensitivity of a photoacoustic signal can be improved, and hence the resultant device can be applied to e.g. a semiconductor device having a rugged surface.
The above mentioned technique is very efficient in that it enables a photoacoustic signal to be detected in a non-contact and non-destruction manner, but also has the following problem. Therefore, if there is a defect such as a crack inside thermal diffusion region Vth 23, the modulated frequency component in the interference intensity signal provides a signal change so that the presence of the defect can be noticed. Thus as shown in FIG. 23 an XY stage shifting signal and an output signal from the lock-in amplifier 16 are processed by a computer 17. Accordingly, the photoacoustic signals corresponding to the respective positions on the sample are displayed on a display (e.g. a monitor television) 18 as image information.
The resolution of a photoacoustic signal in both transverse and depth directions, if the light absorption region Vop 21, i.e. the spot diameter of laser light is smaller than the thermal diffusion region Vth 23, is defined as the thermal diffusion length .mu.s 23. This .mu.s can be defined by Equation (1) ##EQU1## where
k:thermal conductivity of a sample
.rho.:density
c:specific heat
f:intensity-modulation frequency of a laser
For example, when f=10 kHz, Si or Al has a .mu.s of .about.50 .mu.m, and SiO.sub.2 has a .mu.s of .about.5 .mu.m.
Now if the thermal diffusion length 23a as shown in FIG. 24A is formed at a certain modulation frequency, the resolutions in the depth direction and the transverse (horizontal) direction are given as .mu.SD.apprxeq. .mu.s, and .mu.SH.apprxeq.2 .mu.s, respectively. In order to obtain inside information at a deeper position, as understood from Equation (1), the modulation frequency f must be decreased to increase the thermal diffusion length .mu.s as shown in FIG. 24B. As a result, however, the transverse direction resolution .mu.SH and the depth direction resolution .mu.SD will be reduced. Specifically, if in the prior art, in order to obtain the information in the depth direction, the laser modulation frequency f is varied thereby to vary the thermal diffusion length .mu.s, the resolutions in the transverse and depth directions will be disadvantageously varied. Particularly, in order to obtain the information at a deeper position, the thermal diffusion length .mu.s must be further increased to reduce the resolutions. Thus, under the present conditions, it is very difficult to detect information inside of a sample with a miniaturized structure in the order of magnitude of .mu.m.